LIBRARY 

OF  THE 


UNIVERSITY  OF  CALIFORNIA. 


GIF"T  OB" 


C/ass 


THE  1900  SOLAR  ECLIPSE 
EXPEDITION 


OF  THE 


ASTROPHYSICAL    OBSERVATORY 


OF  THE 


SMITHSONIAN    INSTITUTION, 


BY 


S.  P.  LANGLEY,  Director, 

Aided  by  C.  G.  ABBOT. 


(No.  1439.) 

WASHINGTON: 

GOVERNMENT    PRINTING    OFFICE. 
1904. 


THE  1900  SOLAR  ECLIPSE 
EXPEDITION 


OF  THE 


ASTROPHYSICAL    OBSERVATORY 


OF  THE 


SMITHSONIAN    INSTITUTION. 


BY 


S.  P.  LANGLEY,  Director, 

Aided  by  C.  G.  ABBOT. 


( No.  1439.) 

WASHINGTON: 

GOVERNMENT    PRINTING    OFFICE. 
1904. 


,OO 


Y 


THE  SOLAR  ECLIPSE  EXPEDITION 

OF  THE 

ASTROPHYSICAL  OBSERVATORY  OF  THE  SMITHSONIAN  INSTITUTION, 

MAY,     19OO. 


CHAPTER   I. 


OBJECTS,  AND   PREPARATION   FOR  THE   EXPEDITION. 

The  path  of  the  total  solar  eclipse  of  May  28,  1900,  was  unusually  accessible 
from  Washington.  Preliminary  observations  by  the  United  States  Weather  Bureau 
indicated  a  fair  prospect  of  good  observing  weather  along  the  middle  portion  of  the 
belt  of  totality  within  the  United  States.  As  the  conditions  on  this  occasion  were  so 
favorable  to  observation,  it  seemed  desirable  to  send  observers  from  the~8mithsonian 
Astrophvsical  Observatory;  and  Congress  was  therefore  asked  to  appropriate  the  sum 
of  84,000  to  defray  the  expenses  of  the  expedition,  and  this  sum  was  so  appropriated 
and  made  immediately  available." 

1.  OBJECTS  OF  THE   EXPEDITION. 

STRUCTUKE  OF  THE  INNER  CORONA. 

Although  many  excellent  photographs  of  the  corona  had  been  taken  in  recent 
years,  notably  by  Schaberle  and  by  Campbell  of  the  Lick  Observatory,  none  of  them 
had  exhibited  adequately  the  very  curious  and  interesting  structure  which  the  writer 
and  others  had  observed  close  to  the  sun's  limb  during  the  eclipse  of  1878.''  In  the 
hope  of  obtaining  material  for  a  study  of  this  peculiar  structure  of  the  inner  corona, 
its  photography  upon  a  greater  scale  than  any  hitherto  attempted  was  decided  upon 
as  a  prominent  object  of  the  expedition. 

"Observation  of  eclipse  of  May  twenty-eighth,  nineteen  hundred:  For  cost  of  apparatus,  transportation  of 
observers  and  equipment,  subsistence,  reduction  of  observations,  printing  and  publishing  of  results,  not  exceeding 
one  thousand  five  hundred  copies,  and  employment  of  such  temporary  aid  as  may  be  required,  including  all 
necessary  field  and  other  expenses,  four  thousand  dollars. — Deficiency  act,  February  9,  1900. 

bSee  Report  of  S.  P.  Langley,  Washington  Astronomical  and  Meteorological  Observations,  1876,  pt.  2,  Vol.  XXIII, 
p.  203. 

3 

130164 


4  THE    1900    SOLAR    ECLIPSE    EXPEDITION. 

Before  procuring  the  apparatus  for  this  purpose,  several  astronomers  of  great 
experience  were  consulted.  It  was  found  to  be  the  consensus  of  opinion  that  a  lens  of 
both  larger  aperture  and  longer  focus  than  that  used  by  Campbell  in  1898  would  be 
required,  and  I  was  upon  the  point  of  ordering  such  an  one  of  8  inches  aperture  and 
60  feet  focus,  when  Prof.  E.  C.  Pickering,  the  director  of  Harvard  College  Observatory, 
very  generously  offered  me  the  loan  of  a  lens  of  12  inches  aperture  and  135  feet  focus, 
together  with  other  auxiliary  apparatus.  So  welcome  an  offer  could  not  but  make 
the  attainment  of  this  object  of  the  expedition  paramount  over  all  other  considerations, 
and  when  in  addition,  through  Prof.  C.  A.  Young  and  the  trustees  of  Princeton  Uni- 
versity, the  use  of  a  5-inch  lens  of  38  feet  focus  was  secured,  similar  to  that  used  on 
several  occasions  by  Lick  Observatory  eclipse  expeditions,  it  was  apparent  that  in 
optical  equipment  for  photographing  the  inner  corona  the  expedition  would  be  well 
provided. 

VISUAL  OBSERVATIONS. 

To  supplement  this  special  photographic  study  of  the  inner  corona,  the  writer 
desired  to  have  eye  observations  made  in  order  to  see  if  the  filamentary  appearance 
would  be  as  well  marked  as  in  1878.  He  therefore  felt  great  gratification  in  accepting 
the  loan  through  Prof.  S.  J.  Brown,  astronomical  director  of  the  United  States  Naval 
Observatory,  of  the  identical  5-inch  telescope  which  he  had  used  in  the  observations 
on  Pikes  Peak  in  1878.  Two  other  visual  telescopes  of  3  and  7£  feet  focus  were  also 
used.0 

PHOTOGRAPHY  OF  THE  OUTER  CORONA. 

To  photograph  the  outer  corona  several  cameras  of  moderate  focal  length  were 
provided.  Long  exposures  were  intended  for  these,  so  as  to  reach  the  extremest 
limits  of  the  corona  which  could  be  photographed  at  an  eclipse  of  such  short  duration. 

BOLOMETRIC  WORK. 

The  long  experience  of  this  observatory  in  delicate  bolometeric  measures  on  the 
solar  spectrum  suggested  the  advisability  of  an  attempt  to  detect  the  heating  effect 
of  the  coronal  radiations  and,  if  possible,  to  determine  the  distribution  of  their 
energy  in  the  spectrum;  for  if  this  latter  point  could  be  determined,  light  would  be 
thrown  on  the  still  disputed  question  of  the  nature  of  the  corona.  It  will  be 
remembered  that  different  sources  of  radiation  emit  very  unequal  proportions  of 
energy  at  different  wave  lengths,  so  that,  for  example,  if  the  corona  owes  its  light 
to  reflection  of  the  photospheric  radiations  by  particles  of  dust  about  the  sun  an 
energy  spectrum  generally  similar  to  that  of  the  sun  itself  would  be  formed;  but  if 
the  light  is,  on  the  other  hand,  due  to  incandescence  of  such  dust  particles,  owing  to 
their  proximity  to  the  sun,  a  coronal  spectrum  relatively  richer  than  the  sun's  in 

a  The  expedition  is  indebted  to  Mr.  Saegmuller,  of  Washington,  for  the  loan  of  a  mounting  for  one  of  these. 


THE   1900    SOLAR    ECLIPSE    EXPEDITION.  5 

infra-red  rays  would  be  found,  owing  to  the  temperature  of  the  particles  being  less 
than  that  of  the  sun  itself.  Bolometric  study  of  the  corona  was  therefore  included 
among  the  objects  of  the  expedition. 

PHOTOGRAPHIC  SEARCH  FOR  INTRA-MERCURIAL  PLANETS. 

It  was  learned  after  preparations  for  the  eclipse  were  somewhat  advanced 
that  Prof.  W.  H.  Pickering,  of  Harvard  College  Observatory,  intended  to  make 
a  photographic  search  for  as  yet  undiscovered  planets  near  the  sun  by  the  aid 
of  a  special  apparatus  of  his  own  devising.  It  was  further  learned  that  Professor 
Pickering,  and  also  the  eclipse  committee  of  the  Astronomical  and  Astrophysical 
Society  of  America,  desired,  if  possible,  that  a  search  with  similar  apparatus  should 
be  made  by  other  eclipse  parties.  This  observation  was  with  some  hesitancy  added 
to  the  programme,  for  I  had  no  strong  expectation  that  anything  but  negative 
evidence  would  be  secured. 

PHOTOGRAPHY  OF  THE  "FLASH  SPECTRUM." 

In  loaning  the  great  lens  above  referred  to,  the  director  of  the  Harvard  College 
Observatory  suggested  the  use  of  an  objective  prism  with  it  at  the  beginning  of 
totality  for  the  purpose  of  photographing  the  "flash  spectrum,"  and  offered  to  supply 
the  prism  and  an  automatic  apparatus  for  exposing  the  plate  each  second  to  the 
spectrum.  This  proposal  was  accepted,  and  led  to  the  only  attempt  at  photographic 
spectroscopy  included  in  the  programme. 

TIMES  OF  CONTACT. 

It  was  arranged  that  the  observation  of  the  times  of  contact  should  be  made  both 
by  the  usual  method  of  eye  observation  and  by  a  photographic  apparatus. 

GENERAL  SUMMARY  OF  THE  OBJECTS  OF  THE  EXPEDITION. 

Briefly,  the  eclipse  expedition  was  sent  out  from  the  Smithsonian  Astrophysical 
Observatory  for  the  following  purposes: 

1.  To  photograph  and  to   observe  visually  the  minute  structure  of  the   inner 
corona  and  the  prominences. 

2.  To  photograph  the  outer  extensions  of  the  corona  and  possible  intra-mercurial 
planets  or  other  objects  near  the  sun. 

3.  To  measure  the  heating  effect  of  the  inner  coronal  radiations,  and,  if  possible, 
to  determine   the   distribution  of  their    energy  in  the  spectrum  by  the  aid  of  the 
bolometer. 

4.  To  photograph  the  flash  spectrum. 

5.  To  determine  the  times  of  contact. 


6  THE    1900   SOLAR    ECLIPSE    EXPEDITION. 

2.   PREPARATION    FOR   THE   EXPEDITION, 

(a)  SELECTION  OF  SITE. 

The  most  accessible  point  on  the  belt  of  totality  was  Norfolk,  Virginia,  and  here 
also  the  duration  of  totality  was  a  few  seconds  longer  than  at  points  farther  west. 
But,  on  the  other  hand,  the  special  observations  of  the  Weather  Bureau,  extending  over 
several  years,"  had  showed  that  the  chances  of  good  observing  weather  at  points  near 
the  coast  were  inferior  to  those  in  the  inland  districts,  and  rose  to  a  maximum  for 
portions  of  the  States  of  Georgia  and  Alabama.  Nevertheless  the  reports  from  a  few 
stations  in  North  Carolina  came  well  up  with  the  best  Georgia  records. 

The  advantages  of  accessibility,  duration  of  totality,  and  height  of  sun  during 
totality,  all  weighing  in  favor  of  eastern  stations,  with  only  slight  and  not  very  certain 
chances  of  better  weather  as  an  inducement  to  go  farther  to  the  south  and  west,  I 
decided  to  send  Mr.  Abbot,  aid  acting  in  charge  of  the  Astrophysical  Observatory, 
to  look  over  the  ground  and  report  recommending  some  station  in  North  or  South 
Carolina.  He  crossed  the  State  of  North  Carolina,  and  being  convinced  that  there 
would  >>e  no  appreciable  gain  by  going  into  South  Carolina,  he  recommended  as 
alternatives  the  towns  of  Wintou  and  Wadesboro,  North  Carolina,  situated  respectively 
in  the  northeastern  and  south  central  portions  of  the  State.  According  to  the  Weather 
Bureau  bulletin  the  chances  of  fair  weather  at  Wadesboro  were  almost  5  to  1,  and 
were  somewhat  better  than  those  recorded  for  Winton.  Besides  this  difference  for  the 
towns  themselves,  the  data  for  points  in  their  near  neighborhoods  indicated  a  decided 
preference  for  Wadesboro,  and  with  this  in  mind  I  finally  selected  it. 

The  ground  occupied  was  a  sod-covered  level  field  of  several  acres  extent,  on  the 
edge  of  the  town,  which  was  most  cordially  placed  at  the  disposal  of  the  Smithsonian 
Institution  and  its  guests  by  its  owner,  John  Leak,  esq.,  of  Wadesboro. 

Mr.  Abbot  visited  Wadesboro  early  in  April  to  lay  off  the  ground,  and  arranged 
with  Mr.  William  Brasington,  a  local  contractor,  to  erect  a  shed  for  the  apparatus, 
and  to  provide  material  required  for  piers  and  support  of  apparatus.  Accommoda- 
tions were  engaged  for  the  party  at  the  National  Hotel,  Wadesboro.  The  proprietor, 
Mr.  Huntley,  did  his  utmost  to  make  arrangements  for  the  unusual  number  of  these 
and  other  eclipse  guests,  and  at  all  times  cared  for  their  comfort. 

(6)  PERSONNEL  OF  THE  EXPp;DITION. 

The  staff  of  the  Astrophysical  Observatory  available  for  the  expedition  consisted 
of  three  persons  besides  the  writer,  namely:  Mr.  C.  G.  Abbot,  aid  acting  in  charge, 
Mr.  F.  E.  Fowle,  jr.,  junior  assistant,  and  Mr.  A.  Kramer,  instrument  maker.  The 
observing  force  was  increased  by  special  detail,  invitation,  and  volunteering  as  follows: 
Mr.  T.  W.  Smillie,  for  many  years  photographer  of  the  National  Museum,  was  placed 


"Bulletin  No.  27.,  W.  B.,  No.  209.,  U.  8.  Department  of  Agriculture,  Weather  Bureau. 


THE    1900    SOLAR    ECLIPSE     EXPEDITION.  7 

in  cnarge  of  the  photographic  work,  and  Mr.  De  Lancey  Grill,  of  the  Bureau  of  Ameri- 
can Ethnology,  was  detached  to  be  photographic  assistant  to  Mr.  Smillie.  Mr.  C.  W. 
Smith,  a  carpenter,  was  sent  from  the  Smithsonian  Institution.  At  my  request  the 
Superintendent  of  the  United  States  Coast  and  Geodetic  Survey  detailed  Assistant 
Gr.  R.  Putnam  (since  placed  in  charge  of  the  survey  of  the  Philippine  Islands),  to 
determine  the  latitude  and  longitude  of  the  camp  and  assist  in  the  orientation  of  appa- 
ratus, in  which  respect  his  services  were  most  acceptable,  as  well  as  to  observe  visually 
on  the  day  of  the  eclispe.  The  following  five  gentlemen  joined  the  expedition  by 
invitation:  The  Rev.  Father  Searle,  C.  S.  P  ,  with  the  Rev.  Father  Woodman,  C.  S.  P., 
from  the  observatory  of  the  Catholic  University  of  America;  C.  E.  Mendenhall,  Ph.  D., 
then  instructoi  in  physics  at  Williams  College;  Mr.  R.  C.  Child,  for  several  years 
connected  with  the  Astrophysical  Observatory,  but  more  recently  employed  in  the 
United  States  Patent  Office ;  and  Mr.  P.  A.  Draper,  of  Washington.  On  the  day  of  the 
eclipse  two  volunteers,  Mr.  Little,  of  Wadesboro,  and  Mr.  W.  J  Hoxie,  of  Beaufort, 
South  Carolina,  assisted  Mr.  Putnam  in  giving  signals  and  recording  observations, 
while  Mr.  George  Wells,  of  the  Smithsonian  Institution,  recorded  the  observations  of 
the  writer. 

Mr.  C.  D  Walcott,  Director  of  the  United  States  Geological  Survey;  Mr.  R. 
Rathbun,  assistant  secretary  of  the  Smithsonian  Institution;  Mr.  W.  N.  Bannard,  of  the 
Pennsylvania  Railroad;  and  Mr.  J.  E.  Watkins,  of  the  National  Museum,  accompanied 
the  writer  to  Wadesboro  two  days  before  the  eclipse,  and  remained  as  interested 
spectators  and  observers  of  the  general,  aspects  of  the  phenomenon. 

RELATIONS   WITH    THE   YEKKES    OBSERVATORY    EXPEDITION. 

Several  months  prior  to  the  eclipse  an  invitation  was  extended  to  Prof.  G.  E. 
Hale,  director  of  the  Yerkes  Observatory,  to  take  part  as  a  member  of  the  Smithsonian 
expedition.  This  invitation  he  at  first  was  inclined  to  accept,  and  provision  was  made 
for  his  use  of  two  beams  of  light  from  coelostat  mirrors  attached  to  a  second  axis 
added  to  the  great  Grubb  siderostat.  A  little  later,  finding  that  the  Yerkes  Observa- 
tory itself  would  be  able  to  observe  the  eclipse  independently,  Professor  Hale  with- 
drew as  a  member  of  the  Smithsonian  expedition,  but  as  the  two  parties  were  located 
close  together,  he  was  still  able  to  make  use  of  the  coelostat  beams  above  mentioned. 
Much  benefit  was  derived  from  the  counsel  and  friendly  interest  of  the  director  and 
members  of  the  Yerkes  Observatory  expedition,  thus  so  fortunately  located  near  that 
of  the  Smithsonian  Institution. 

(e)  TRANSPORTATION. 

The  plans  of  the  observations  had  so  far  outgrown  what  was  intended  when  the 
small  appropriation  was  asked  for  that  this  proved  hardly  sufficient,  with  strictest 
economy,  to  defray  all  the  expenses.  In  these  circumstances  I  felt  justified  in  asking 


8  THE    1900    SOLAR    ECLIPSE    EXPEDITION. 

the  cooperation  of  the  several  railroads  whose  tracks  should  be  traversed  in  the 
journey.  They  generously  responded,  offering  each  and  all  free  transportation 
both  for  observers  and  freight.  I  take  this  opportunity,  therefore,  to  acknowledge 
the  indebtedness  of  the  expedition  to  the  Pennsylvania,  the  Richmond,  Fredericks- 
burg  and  Potomac,  the  Atlantic  Coast  Line,  and  the  Seaboard  Air  Line  Railroad 
companies  for  the  free  transportation  of  observers  and  equipment  both  going  and 
returning  over  the  portions  of  the  route  included  by  their  several  lines. 

(d)  PRELIMINARY  PRACTICE  AT  WASHINGTON. 

There  can  be  no  doubt  that  one  of  the  first  essentials  to  success  in  eclipse 
observations  is  adequate  preliminary  practice;  for  the  time  of  totality  is  too  short  for 
thinking,  and  the  mind  is  too  much  disturbed  by  the  feeling  of  necessity  for  haste  to 
do  rightly  anything  which  has  not  become  a  settled  habit.  In  order  that  there 
should  be  the  greatest  possible  opportunity  for  practice  in  the  use  of  the  most 
important  pieces  of  apparatus,  and  a  chance  to  make  such  modifications  as  actual 
trial  might  suggest  while  all  the  mechanical  facilities  were  at  hand,  I  gave  especial 
attention  to  a  field  installation  for  rehearsal,  which  was  made  in  April  by  setting  up 
the  apparatus  in  the  Smithsonian  Park. 

A  certain  day  and  hour  was  selected  at  which  an  imaginary  eclipse  was  to  occur. 
The  Brashear  coelostat,  the  Grubb  siderostat,  the  portable  photographic  house,  the 
12-inch  lens  of  135-foot  focus,  and  the  5-inch  lens  of  38-foot  focus,  with  the  cloth 
tubes  leading  from  the  lenses  to  the  photographic  house,  were  all  adjusted  for  the 
supposed  eclipse.  Signals  were  given  in  warning,  and  at  the  time  of  assumed  totality 
actual  shifting  of  the  plate  holders,  as  if  for  the  photographic  exposures,  was  gone 
through  with.  The  assumed  eclipse  being  more  accommodating  than  a  real  one,  it 
repeated  itself  a  few  minutes  later  to  give  a  further  opportunity  for  practice  on  the 
part  of  the  observers.  As  a  preliminary  rehearsal  the  whole  went  off  very  well. 

At  the  conclusion  of  the  rehearsal,  which,  together  with  the  preparations  for  it, 
lasted  for  several  days,  the  apparatus  and  accessories  were  immediately  taken  down 
and  packed  for  shipment  to  Wadesboro.  A  freight  car  filled  to  the  roof  with  the 
equipment  started  from  Washington  on  May  4  and  reached  Wadesboro  on  May  9. 


CHAPTER  II. 


1.  ESTABLISHMENT  OF  THE  CAMP. 

A  few  days  after  the  shipment  of  the  apparatus  the  advance  guard  of  the  party, 
consisting  of  Messrs.  Abbot,  Fowle,  Kramer,  Draper,  and  Smith,  went  on,  carrying 
with  them  the  galvanometer  for  the  bolometric  work,  and  the  chronometer  and  other 
delicate  pieces  of  apparatus.  Immediately  upon  its  arrival  the  carload  of  equipment 
was  removed  to  the  camp,  shown  in  PI.  II,  which  had  meantime  been  made  ready 
for  occupancy  under  the  good  auspices  of  Mr.  Brasington.  From  then  till  after  the 
eclipse  day  and  night  were  filled  with  preparation. 

Mr.  Putnam,  of  the  Coast  and  Geodetic  Survey,  reaching  Wadesboro  May  10, 
immediately  established  his  meridian  instrument,  and  the  weather  continuing  fine 
he  was  able  to  get  good  observations  for  latitude  and  longitude  on  sevei'al  nights. 
During  the  days  he  assisted  to  adjust  the  axes  of  the  Grubb  siderostat  and  the 
Brashear  equatorial  and  coelostat,  and  ran  north  and  south  and  east  and  west  lines, 
and  independently  computed  and  ran  lines  parallel  to  the  direction  of  the  coelostat 
beam  and  the  azimuth  of  the  40-foot  direct  telescope  axis,  so  that  his  work  during 
the  few  days  that  he  remained  at  Wadesboro  was  most  valuable. 

Mr.  Putnam  returned  shortly  before  the  eclipse  and  made  additional  observations 
for  position.  The  final  values  obtained  for  the  latitude  and  longitude  of  the  coast 
survey  tent  shown  in  the  diagram  of  the  camp  (PI.  Ill)  were: 

Latitude         34°  57'    52"       North. 

(  80°  04'    27"     ) 
Longitude  j    ^   g™,  ^  o8  >  West  of  Greenwich. 

On  May  16  Messrs.  Smillie,  Mendenhall,  Child,  and  Gill  came,  and  the  rest  of  the 
observers  arrived  on  Friday  and  Saturday,  May  25  and  26. 

Meanwhile  members  of  the  Yerkes  Observatory  expedition  had  arrived,  and  as 
Professor  Hale  was  to  make  use  of  two  mirrors  mounted  on  a  second  axis  connected 
with  the  Grubb  siderostat,  their  camp  was  laid  out  immediately  adjoining  ours  to  the 
west. 

From  the  first  Mr.  Brasington,  the  contractor,  entered  into  the  plans  of  both  camps 
with  hearty  interest,  and  his  aid  was  most  valuable  in  supplying  everything  in  his 
line,  from  bricks  and  lumber  for  the  piers  and  building  to  barrels  for  reservoirs  and 
108A— 04 2  9 


10  THE    1900    SOLAR    ECLIPSE    EXPEDITION. 

washing  tanks.  He  must  have  given  up  all  his  other  work  and  devoted  himself 
and  his  force  of  men  wholly  to  the  eclipse,  for  besides  the  Smithsonian  and  Yerkes 
expeditions,  Professor  Young  and  his  party  from  Princeton,  and  the  English  expedi- 
tions of  the  Reverend  Mr.  Bacon,  and  Mr.  Nevil  Maskelyne,  had  all  come  to  Wades- 
boro  and  found  in  Mr.  Brasington  their  mainstay  and  ready  helper  for  all  demands. 


RELATIONS   WITH    THE    TOWNSPEOPLE. 


The  townspeople  of  Wadesboro  manifested  a  lively  interest  in  the  eclipse 
preparations;  and  not  only  were  they  interested,  but  helpful,  and  they  treated  the 
observers  with  marked  cordiality.  At  one  of  the  first  preliminary  visits  of  Mr.  Abbot 
Judge  Bennett,  a  prominent  citizen  and  ex-Representative  to  Congress,  called  upon 
him  and  tendered  any  assistance  in  his  power  to  give — an  example  followed  by 
others.  After  the  arrival  of  the  parties  a  meeting  was  held  by  the  townspeople  and 
a  committee  appointed  to  assist  the  astronomers  in  any  possible  way.  As  a  further 
act  of  hospitality  several  carriages  were  placed  at  the  disposal  of  such  of  the  guests 
as  chose  to  ride;  and  finding  that  in  the  press  of  preparation  this  kindly  offer  could 
not  be  improved  so  much  as  the  people  could  have  liked,  the  carriages  were  made 
to  call  regularly  to  convey  the  observers  to  and  from  the  hotels  to  the  camps, 
morning,  noon,  and  night.  Along  with  this  public  cordiality,  private  hospitalities 
were  not  wanting,  and  one  and  all  of  those  who  observed  at  Wadesboro  must  join 
in  acknowledgment  of  the  courtesies  received. 

As  some  slight  return  one  or  more  of  the  Smithsonian  party  was  usually  in 
attendance  at  the  5-inch  visual  telescope  on  fair  nights  to  show  celestial  objects  to 
such  as  wished  to  look,  and  quite  a  number  of  the  people  gathered  there  every  night  and 
seemed  to  enjoy  the  opportunity. 

Provision  was  made  by  the  town  authorities  for  effectually  guarding  the  grounds 
from  intrusion  on  the  day  of  the  eclipse.  The  whole  space  was  roped  off,  the  nearest 
street  closed,  and  special  constables  prevented  all  approach  until  after  the  eclipse  had 

passed. 

2.  APPARATUS   AND  ACCESSORIES. 

The  location  of  the  apparatus  at  the  camp  is  shown  in  PL  III.  On  the  left  of  the 
"wagon  track"  is  the  main  portion  of  the  Yerkes  camp.  All  at  the  right  of  the  "wagon 
track"  appertains  to  the  Smithsonian  expedition,  with  the  exception  of  the  places 
marked  "spectroscope,"  "concave  mirror,"  and  "bolometer  house,"  and  those  marked 
"shadow  band  screen"  and  "meteorological  station."  The  first  three  of  these  were 
under  Professor  Hale's  immediate  charge,  and  were  supplied  with  beams  of  light  from 
the  Grubb  siderostat,  while  the  two  last  named  were  occupied  Toy  Mr.  Clayton  of  the 
Blue  Hill  Meteorological  Observatory. 

Passing  now  to  the  Smithsonian  camp  proper,  there  is  shown  at  the  northwest 
corner  the  Brashear  equatorial  and  coelostat,  and  immediately  in  front  the  12-inch 


THE   1900   SOLAR    ECLIPSE    EXPEDITION.  11 

Harvard  lens  and  objective  prism  through  which  the  beam  from  the  coelostat  passed 
into  the  two-branched  camera  tube.  The  direct  beam  through  the  lens  came  to  focus 
in  the  small  dark  room,  within  which  the  operator  changed  plates  and  made  the 
exposures  by  a  cord  operating  a  shutter  at  the  lens.  Immediately  before  and  after 
totality  the  prism  was  to  be  interposed  just  in  front  of  the  lens  to  catch  the  "flash 
spectrum"  1o  be  thrown  down  the  northerly  branch  tube  to  a  photographic  plate 
moved  each  second  by  the  automatic  plate  carrier. 

On  the  right  of  the  dark  room  is  seen  projecting  easterly  the  inclined  tube  of  the 
5-inch  Princeton  lens.  This  lens  and  its  tube  were  fixed  permanently  at  an  inclina- 

\ 

tion  suitable  to  the  position  of  the  sun  on  the  moment  of  totality;  and  the  image  was 
received  on  a  moving  plate  holder  driven  by  a  water  clock  within  the  dark  room. 
An  observer  within  the  dark  room  changed  plates  and  operated  the  exposing  shutter 
by  a  cord. 

Under  a  little  shelter  on  the  north  of  the  dark  room  was  a  chronograph  and 
chronometer,  from  which  electric  circuits  ran  to  the  shutters  of  the  12-inch  and  5-inch 
lenses,  to  the  plate  carrier  of  the  prismatic  camera,  to  the  automatic  contact  camera 
(shown  north  and  east  of  the  dark  room),  to  the  Coast  Survey  tent  (shown  at  the 
south  of  the  camp),  where  contacts  were  observed  by  Mr.  Putnam,  to  the  SJ-inch 
telescope  where  contacts  were  observed  by  Rev.  Fr.  Woodman,  and  finally  to  the 
Yerkes  camp. 

Nearly  west  of  the  photographic  dark  room  was  the  main  building  containing  the 
bolometric  and  " intramercurial  planet"  photographic  apparatus.  The  bolometric 
apparatus  was  scarcely  less  thoroughly  installed  than  when  permanently  in  use  in 
Washington.  It  was  contained  within  a  double-walled  inner  chamber.  The  galva- 
nometer rested  on  a  mercury  float,  itself  supported  on  a  system  of  stone  and  rubber 
blocks,  which  in  turn  rested  upon  a  heavy  and  deeply  bedded  pier.  At  a  later  page 
the  bolometric  arrangements  will  be  more  fully  described. 

Within  the  bolometer  chamber  is  shown  the  clock  used  to  drive  the  intra- 
mercurial planet  apparatus.  This  clock"  is  the  one  customarily  used  to  drive  the 
spectro-bolometric  train  at  Washington,  and  was  placed  within  the  bolometric  chamber, 
as  shown,  in  order  to  utilize  its  outside  gearing  with  hand  driving  for  producing  a 
holograph  of  the  coronal  spectrum,  if  this  proved  practicable,  at  the  same  time  that 
the  clock  itself  was  driving  the  large  camera. 

The  intramercurial  planet  apparatus  in  the  eastern  wing  of  the  building  consisted 
of  four  cameras  mounted  upon  one  polar  axis.  Of  these,  two  were  of  3  inches  aperture 
and  1 1  feet  focus,  and  designed  to  cover  fields  of  about  1  hour  in  right  ascension  by 
10°  in  declination  east  and  west  of  the  sun,  respectively.  The  two  other  cameras 
had  Ross  lenses  of  4£  inches  aperture  and  39  inches  focus. 

"See  PI.  XIII,  Annals  of  Astrophyaical  Observatory,  Smithsonian  Institution,  Vol.  I. 


12  THE    1900   SOLAR    ECLIPSE    EXPEDITION. 

Between  the  main  building  and  the  Coast  Survey  tent  is  located  the  5-inch  equa- 
torial, and  in  a  line  running  northeast  from  it  were  stationed  a  3  J-inch  telescope  with 
rough  altazimuth  mounting,  ?nd  still  farther  in  the  same  line,  but  beyond  the  dark 
room,  a  6-inch  equatorial  of  7£  feet  focus.  Near  the  CoavSt  Survey  tent  was  located 
the  signal  bell  upon  which  warnings  were  struck  at  the  time  of  the  eclipse,  as  directed 
by  the  observer  within  the  tent. 

Several  of  the  separate  pieces  of  apparatus  are  shown  in  Pis.  IV,  V,  VI,  VII, 
VIII,  IX,  X,  and  XI,  and  these  may  be  consulted  in  connection  with  the  following 
description  of  some  of  the  instruments. 

THE    COMBINED   EQUATORIAL   AND   COELOSTAT. 

(Plate  IV.) 

As  long  ago  as  1880  I  employed  a  12-inch  plane  mirror  on  the  south  end  of  the 
polar  axis  of  the  equatorial  at  Allegheny  Observatory  to  give  a  fixed  solar  beam,  and 
was  accustomed  to  use  it  either  as  a  polar  siderostat  sending  the  beam  down  the  axis, 
or  as  what  is  now  termed  the  coelostat.  In  this  latter  use  the  mirror  lay  in  the  plane 
of  the  polar  axis,  which  was  then  caused  to  rotate  once  in  forty-eight  hours  instead  of 
once  in  twenty-four  hours,  as  usual.  Suppose  the  mirror  to  be  set  so  that  an  observer 
stationed  at  the  east  sees  the  sun's  face  as  it  rises,  he  will  obviously  continue  to  see  it, 
because  the  mirror  turns  half  as  fast  as  the  earth.  I  did  not  then  know  that  this 
simple  arrangement  for  a  fixed  beam  was  in  fact  described  in  1863  by  von  Littrow" 
who  refers  its  discovery  to  August,  nearly  thirty  years  earlier. 

This  instrument  is  so  simple  mechanically  and  so  desirable  in  practice  from  the 
fact  that  it  gives  a  fixed  field  of  view  instead  of  one  rotating  about  a  fixed  central  point 
as  in  other  siderostats,  that  I  determined  to  employ  the  coelostat  in  connection  with 
the  long-focus  lens.6  It  was  desired  also  to  have  an  accurately  driven  equatorial 
mounting  for  a  6-inch  photographic  lens  of  7 £  feet  focus,  and  the  two  requirements  were 
combined  in  the  instrument  shown  in  PI.  IV,  which  was  constructed  for  the  occasion 
by  Brashear.  Two  polar  axes,  one  for  the  equatorial  camera,  the  other  for  the  18-inch 
coelostat  mirror,  are  driven  by  a  common  clock.  The  observer  is  shown  ready  to 
manipulate  both  the  exposure  of  the  camera  and  the  setting  of  the  objective  prism  for 
the  "flash  spectrum,"  both  of  which  duties  were  included  in  his  charge. 

«O.  von  Littrow,  "Wien.  Berichte,  XLVIII,  II,  pp.  337-348,  1863. 

&  As  generally  employed  the  coelostat  reflects  the  beam  in  a  horizontal  direction  to  the  east  or  west  according 
as  the  celestial  object  is  east  or  west  of  the  meridian;  so  that  as  thus  used  the  instrument  is  least  efficient  when 
the  object  is  farthest  from  rising  or  setting,  respectively.  A  second  mirror  has  recently  been  introduced  here  in 
connection  with  the  coelostat  in  such  a  way  as  to  produce  an  efficient  universal  instrument,  which  is  now  (1903) 
proving  useful  in  the  work  of  the  Astrophysical  Observatory,  and  it  is  hoped  may  meet  with  favor  elsewhere.  The 
second  mirror  is  mounted  above  and  south  of  the  coelostat  proper,  upon  a  support  provided  with  slides  in  both  an 
east  and  west  and  north  and  south  direction,  like  the  slide  rest  of  a  lathe.  The  beam  is  sent  north  in  a  horizontal 
direction  from  the  second  mirror.  This  mirror  occupies  different  positions  on  its  north  and  south  track,  according  to 
the  declination  of  the  celestial  object,  and  is  sometimes  shifted  east  or  west  to  avoid  interrupting  the  entering  beam. 
For  the  sun,  and  at  this  latitude,  two  27-inch  mirrors  will  at  the  most  unfavorable  time  give  a  20-inch  circular  beam. 


THE    1900   SOLAR    ECLIPSE    EXPEDITION.  13 

THE   LONG-FOCUS   TELESCOPES. 
(Plates  II,  V,  and  VI.) 

The  tubes  for  the  135-foot  focus  lens  were  42  inches  square,  of  black  canton 
flannel  stretched  by  ropes  running  longitudinally  at  the  four  corners,  and  supported 
on  trestlework.  They  were  provided  with  diaphragms  at  10-foot  intervals  and  were 
covered  over  with  a  canvas  in  the  form  of  a  prolonged  A  tent.  For  the  direct 
photography  30-inch  square  plates  were  used  in  plate  holders  with  roll  shutters.  All 
the  photographic  plates,  specially  prepared  by  the  Cramer  Company,  of  St.  Louis, 
were  double  coated,  extremely  rapid,  and  of  the  so-called  "isochromatic"  preparation. 

The  38-foot  focus  tube  was  composed  of  a  conical  spruce  frame  of  square  cross 
section  covered  with  canvas  blackened  within  and  was  supported  at  the  proper  angle 
on  the  tops  of  posts.  .  The  lens  with  its  adjustments  was  separately  supported  upon  a 
post  8  inches  square  and  braced  well  from  the  sides.  As  has  been  said  the  exposures 
of  both  the  long-focus  cameras  were  made  by  the  aid  of  cords  running  from  the  dark 
room  to  shutters  at  the  lenses.  A  water  clock  with  horizontal  cylinder  was  con- 
structed to  control  the  movement  of  the  11  by  14  inch  plates  along  inclined  guides  at 
the  focus  of  the  38-foot  lens. 

INTRAMERCURIAL   PLANET   CAMERAS. 

(Plate  VII.) 

As  already  stated,  it  had  been  recommended  by  Prof.  W.  H.  Pickering  that  several 
stations  should  be  equipped  with  3-inch  lenses  of  11  feet  focus,  to  be  used  to  photo- 
graph large  fields  of  view  about  the  sun,  in  the  hope  of  discovering  possible  intra- 
mercurial  planets.  In  PI.  VI  the  projecting  box  marked  "1,  2"  is  the  end  of  a  pair 
of  cameras  of  this  type  with  axes  inclined  so  that  together  the  cameras  covered  a  field 
of  view  extending  from  about  one  hour  east  to  one  hour  west  and  from  5°  north  to 
5°  south  of  the  sun.  It  was  intended  to  use  a  nest  of  8  by  10  inch  plates  in  each  of 
these  cameras,  arranged  on  a  curved  surface  so  as  to  be  in  the  best  focus  in  all  parts 
of  the  field,  but  single  plates  24  by  30  inches  were  substituted  finally,  although  as 
the  result  proved  the  focus  was  not  the  best  all  over  their  extent.  Two  other  large 
cameras,  shown  in  PI.  VII,  had  Ross  portrait  lenses  of  4£  inches  aperture  and  40 
inches  focus,  and  these  were  used  with  30-inch  square  plates.  All  these  four  rather 
heavy  cameras  were  mounted  on  a  single  "home-made"  polar  axis,  but  this  was  so 
well  counterbalanced  that  the  clock  drove  it  with  great  ease  and  accuracy. 

THE    AUTOMATIC    RECORDING   CONTACT   CAMERA. 

In  this  instrument  a  22-inch  focus  lens,  with  pin-hole  diaphragm,  formed  the 
solar  image  on  a  15-inch  circular  celluloid  photographic  plate  of  little  sensitive- 
ness, which  was  rotated  by  a  ratchet  wheel  of  168  teeth,  driven  with  an  electric 
escapement.  This  escapement  and  a  shutter  in  front  of  the  lens  were  operated 


14  THE   1900   SOLAR    ECLIPSE    EXPEDITION. 

once  a  second  by  the  break  circuit  chronometer,  but  the  impulses  were  so  timed  by  aid 
of  relays  that  the  shutter  was  always  opened  and  closed  before  the  plate  was  moved 
along  one  tooth.  An  observer  was  charged  with  the  duty  of  cutting  off  exposures 
with  a  card  at  certain  seconds  after  the  minute  break.  In  consequence  of  the  fifty- 
ninth  impulse  being  dropped  by  the  chronometer,  only  59  images  were  made  by  the 
instrument  in  a  minute,  and  as  there  were  168  teeth  on  the  ratchet  wheel  it  is  apparent 
that  more  than  two  and  one-half  minutes  were  consumed  in  one  rotation  of  the  plate. 
During  that  time  the  image  of  the  sun  would  be  carried  so  far  by  the  earth's  diurnal 
motion  that  the  successive  rows  of  images  would  not  overlap,  but  would  be  in  a  spiral. 

THE    BOLOMETKIC   APPARATUS. 
(Plates  VIII  and  IX.) 

The  bolometric  observations  lay  in  a  wholly  untried  field,  and  practically  no 
forecast  could  be  made  of  the  heating  effect  of  the  coronal  radiations  to  be  measured, 
and  hence  provision  was  made  for  a  considerable  modification  of  the  programme, 
according  to  the  results  reached  in  the  first  seconds  of  totality.  A  17-inch  mirror,  on 
the  Grubb  siderostat,  reflected  the  beam  horizontally  south  through  a  cat's-eye 
diaphragm,  whose  aperture  could  be  controlled  by  cords  running  to  a  slider  on  a  scale 
at  the  left  of  the  observer  at  the  galvanometer.  He  could  thus  vary  the  intensity  of 
the  beam  several  thousand  fold.  The  beam  was  concentrated  upon  a  slit  1  centimeter 
high  and  1  millimeter  wide  by  a  concave  mirror  of  50  centimeters  aperture  and  1 
meter  focus,  controlled  by  the  second  observer,  who,  watching  the  image  on  the  jaws 
of  this  slit,  adjusted  the  concave  mirror  so  as  to  throw  the  desired  part  upon  it.  Two 
small  flat  mirrors  beyond  the  slit  reflected  the  beam  to  a  collimating  mirror  of  75 
centimeters  focus,  whence  it  passed  to  the  prism. 

This  was  a  compound  objective  prism,  loaned  by  the  Harvard  College  Observa- 
tory, having  two  circular  half  prisms  rotating  about  a  common  axis,  so  that  their 
combined  angle  could  be  varied  from  0°  to  15°.  This  feature  was  not  used  during 
the  eclipse,  but  previously  the  prisms  were  set  at  an  angle  judged  proper  for 
dispersing  the  coronal  spectrum,  should  this  prove  desirable.  One  of  these  prisms 
was  silvered  on  the  back,  the  intention  being  that  the  combination  could  be  used 
simply  as  a  plane  mirror  in  the  early  part  of  totality  to  get  an  idea  of  the  intensity 
of  the  coronal  radiation,  but  could  later  be  turned  round  so  that  the  collimated 
beam  would  pass  through  the  prism  from  front  to  back  and  return  as  a  spectrum.  In 
either  case  the  rays  next  fell  upon  an  image-forming  mirror  of  75  centimeters  focus 
and  were  brought  to  focus  upon  the  bolometer  strip,  which  was  1  centimeter  high  and 
1  millimeter  wide.  The  warming  of  this  strip  was  to  be  determined  by  the  deflection 
of  the  sensitive  galvanometer,  and  it  was  arranged  that  this  deflection  should  be  read 
visually  and  photographically  recorded,  in  the  manner  described  in  Volume  I  of  the 
Annals  of  the  Astrophysical  Observatory. 


THE    1900    SOLAR    ECLIPSE    EXPEDITION.  15 

Two  observers  were  employed.  One  was  to  read  the  galvanometer  deflections, 
record  all  observations,  and  operate  the  cat's-eye  diaphragm,  while  the  other  was  to 
adjust  the  position  of  the  image  upon  the  slit,  set  the  prism  to  previously  adjusted 
stops  and  clamp  it,  and  to  turn  the  prism  and  recording  plate  by  clockwork  through 
previously  determined  intervals  for  observing  the  energy  spectrum  of  the  corona. 
He  also  had  in  hand  a  pasteboard  shutter  and  a  plate  of  glass,  which  he  was  to  inter- 
pose before  the  bolometer  at  certain  times  to  give  readings,  corresponding  to  zero  of 
outside  radiations  upon  the  bolometer,  and  to  give  an  idea  of  the  amount  of  radiations 
transmissible  by  glass,  respectively. 

3.  ASSIGNMENT  OF  OBSERVERS  AND  PRELIMINARY  REHEARSALS. 

The  disposal  of  the  observing  force  was  as  follows : 

Mr.  S.  P.  Langley,  Director,  telescopic  observations  with  the  5-inch  equatorial. 

Mr.  C.  G.  Abbot,  aid  acting  in  charge,  manipulation  of  the  spectrobolometer. 

Mr.  T.  W.  Smillie,  in  general  charge  of  photographic  work,  manipulation  of  the 
135-foot  focus  camera, 

Mr.  F.  E.  Fowle,  jr.,  manipulation  of  the  38-foot  focus  camera. 

The  Rev.  Father  Searle,  C.  S.  P.,  in  general  charge  of  intra-mercurial  planet 
apparatus,  manipulation  of  the  11 -foot  focus  cameras. 

Mr.  G.  R.  Putnam,  determination  of  times  of  contact  and  direction  of  warning 
signals. 

Mr.  C.  E.  Mendenhall,  observing  at  the  galvanometer. 

Mr.  R.  C.  Child,  telescopic  observations  with  6-inch  equatorial  and  in  charge  of 
electric  circuits,  including  especially  the  prismatic  camera  and  the  automatic  contact 
camera. 

The  Rev.  Father  Woodman,  C.  S.  P.,  times  of  contact  and  general  observations 
with  3^-inch  telescope. 

Mr.  Delancey  Gill,  manipulation  of  7^-foot  focus  camera  and  the  objective  prism. 

Mr.  P.  A.  Draper  and  Mr.  C.  W.  B.  Smith,  exposures  of  the  two  40-inch  cameras 

Mr.  A.  Kramer,  instrument  maker,  looking  to  movements  of  the  siderostat  and 
the  5-inch  equatorial. 

Two  volunteers,  Mr.  Little,  of  Wadesboro,  and  Mr.  Hoxie,  of  Beaufort,  South 
Carolina,  were  assigned,  respectively,  to  strike  signals  and  to  record  times  of  contact 
under  direction  of  Mr.  Putnam. 

On  the  two  days  next  preceding  the  eclipse,  several  full  rehearsals  were  carried 
through  to  familiarize  all  the  observers  with  their  duties,  which  were  specified  in 
written  directions  furnished  each  observer,  that  nothing  might  be  overlooked. 


CHAPTER 


OBSERVATIONS  AND   RESULTS. 

The  day  of  the  eclipse  proved  cloudless;  the  sky,  while  not  of  the  deepest  blue, 
was  yet  more  than  ordinarily  clear,  and  the  observing  programme  was  carried  through 
with  general  success.  In  what  follows,  the  observations  will  be  described  under  these 
four  heads:  (1)  General  and  telescopic  observations;  (2)  Photography  of  the  inner 
corona  and  prominences;  (3)  Photography  of  the  outer  corona  and  outlying  regions 
of  sky;  (4)  Bolometric  observation  of  the  corona. 

1.  GENERAL  AND  TELESCOPIC  OBSERVATIONS. 
(a)  GENERAL  OBSERVATIONS. 

The  degree  of  darkness  during  totality  was  considered  by  several  to  be  about 
the  same  as  on  a  clear  night  at  full  moon,  but  of  planets  and  stars  only  a  few  of  the 
brightest  were  noted.  Shadow  bands  were  seen  by  several  observers,  but  not  very 
distinctly.  Mr.  Child  reported  a  pronounced  pulse  of  light  and  shade  eight  minutes 
before  totality.  The  cessation  of  the  noises  of  birds  and  animals  as  the  time  of 
totality  drew  near  was  remarked.  The  equatorial  extensions  of  the  corona  on  either 
side  of  the  sun  were  variously  estimated  at  from  2  to  5  lunar  diameters  by  naked-eye 

observers. 

(6)   COLOR  OF  THE  CORONA. 

Mr.  Child,  an  artist,  noted  a  distinct  apple-green  tinge  to  the  outer  coronal  light, 
which  held  to  within  a  half  diameter  of  the  limb,  where  the  color  began  to  grow  more 
yellow,  the  extreme  inner  coruna  appearing  to  him  of  a  pale  golden  hue.  Others 
likened  the  coloring  to  that  of  mother-of-pearl,  but  nearly  all  noted  a  distinct  yellow- 
ish tinge.  One  observer  reported  a  ruddier  color  toward  the  outer  end  of  the  stream- 
ers on  the  lower  side  of  the  sun,  but  several  noted,  on  the  contrary,  a  tendency 
toward  green  at  increasing  distances  from  the  limb. 

(c)  TELESCOPIC  VIEW. 

To  the  writer's  view  with  the  5-inch  telescope  the  inner  corona  was  filled  with 
detail,  but  far  less  sharp  and  definite  than  he  saw  it  on  Pikes  Peak  in   1878.     He 
could  not  identify  any  connection  between  the  coronal  structure  and  the  presence  of 
108A— 04 3  17 


18 


THK    1900    SOLAR    ECLIPSE    EXPEDITION. 


prominences,  while  his  impression  was  that  the  details  contained  more  ogivai  curves 
than  straight  streamers.  Having  in  mind  the  wonderful  structure  seen  with  the  same 
instrument  in  the  clear  mountain  air  twenty-two  years  before,  *  the  impression  was  a 
disappointing  one. 

Mr.  Child  and  the  Rev.  Father  Woodman  noted  the  appearance  of  Bailey's  beads 
very  distinctly,  and  the  former  made  sketches  from  which  he  later  prepared  a  repre- 
sentation of  the  corona  in  pastel  colors. 

(d)  TIMES  OF  CONTACT. 

Times  of  contact  were  observed  by  Mr.  Putnam  and  by  the  Rev.  Father  Wood- 
man by  eye  and  ear  methods,  and  were  also  signaled  to  the  chronograph.  Plate  X 
shows  the  Coast  and  Geodetic  Survey  tent  as  adapted  for  the  former's  observations 
on  the  day  of  the  eclipse.  Owing  to  the  imperfect  working  of  Mr.  Putnam's  circuit, 
his  chronograph  record  was  lost,  but  Mr.  Hoxie,  recording  for  Mr.  Putnam,  noted  the 
instant  of  his  key  taps  as  well  as  the  results  of  his  eye  and  ear  observations.  The 
following  table  gives  the  mean  of  these  two  records.  Third  contact,  as  given  below, 
was  not  observed  in  the  telescope  by  Mr.  Putnam,  as  he  was  hindered  by  giving 
signals,  but  Mr.  Hoxie  recorded  the  flash  of  light  on  the  chronometer  face.  Rev. 
Father  Woodman's  observations  were  recorded  by  him  with  a  pocket  chronometer, 
and  his  signals  also  appear  on  the  chronograph  sheet;  and  since  there  is  a  slight 
discrepancy  both  records  are  given.  The  times  given  below  depend  on  three  chro- 
nometers compared  for  several  days  with  telegraphic  noon  signals  supplied  from 
Washington.  A  further  check  was  given  by  transit  observations  on  the  night  after 
the  eclipse. 


Number  of  contact. 

Mean  time  for  seventy-fifth  meridian. 

Mr.  Putnam. 

Rev.  Father  Woodman. 

First   

19"     36™    19'.  7 
20      45     15  .  5 
20      46     44  .  1 
22      05     37  .  3 

Jly  pocket  chro- 
nometer. 

19h     36™           21" 
20      45            16 
20      46            47 
22      05            26 

By  chrono- 
graph sheet. 

22'.  0 
16.4 
43.5 

27.8 

Second  . 

Third 

Fourth 

The  record  of  the  automatic  contact  camera  proved  of  little  value;  for  at  first 
and  fourth  contacts  there  is  an  uncertainty  of  ten  seconds  just  when  the  indentation  of 
the  image  ceases;  second  contact  was  lost  by  failure  of  the  electrical  circuit,  and  five 
seconds  elapsed  after  third  contact  before  the  images  became  strong  enough  to  be 
distinguishable.  The  record  is,  however,  so  striking  that  a  reproduction  of  the 
negative  which  includes  totality  is  here  given.  (PI.  XI.) 


a  Washington  Astronomical  and  Meteorological  Observations,  Vol.  XXIII,  pt.  2,  p.  203,  1876. 


THE   1900   SOLAR    ECLIPSE    EXPEDITION. 


19 


A  record  of  third  contact  is  practically  furnished  by  the  chronograph  sheet  which 
recorded  the  exposures  of  the  38-foot  camera,  for  the  negative  corresponding  to  an 
exposure  terminating  at  20h  46m  43". 7  shows  a  thin,  bright  crescent  streak  inside  the 
very  black  overexposed  prominence  layer.  Therefore  at  the  time  noted  a  portion  of 
the  sun  had  emerged  which  was  sufficiently  bright  to  produce  a  positive  effect  on  the 

plate. 

2.  PHOTOGRAPHY  OF  THE  INNER  CORONA  AND  PROMINENCES. 

(a)  THE  135-FOOT  CAMERA. 

Mr.  Smillie  secured  five  negatives  during  totality,  which  were  given  exposures  of 
|,  1,  4,  16,  and  8  seconds,  respectively,  the  last  exposure  ending  a  little  after  third 
contact.  Owing  to  a  failure  of  the  electric  circuit,  the  exact  times  of  exposure  were 
not  recorded  on  the  chronograph,  as  had  been  intended,  but  all  the  negatives  are 
excellent  and  show  both  the  inner  corona  and  the  prominences  with  much  detail. 
The  originals  must  be  seen  to  be  appreciated,  for  there  is  no  way  of  reproducing  on 
the  printed  page  what  can  be  seen  on  looking  through  a  glass  negative  against  a 
ground-glass  surface.  Nevertheless,  where  there  is  so  much  of  interest  in  the  original 
a  good  deal  remains  in  a  good  reproduction,  so  that  the  series  of  Pis.  XII  to  XX  are 
given  in  the  hope  of  conveying  to  the  reader  something  of  the  structure  of  the  inner 
corona  and  prominences  at  the  time  of  the  eclipse. 

Pis.  XII  to  XIX,  inclusive,  show  the  whole  corona  as  photographed  near 
midtotality  with  sixteen  seconds  exposure  and  PI.  XX  from  an  exposure  of  eight 
seconds  ending  at  third  contact  gives  the  best  view  obtained  of  the  great  prominences 
on  the  southwest  quadrant.  The  positions  indicated  are  determined  as  follows :  From 
the  center  of  the  sun's  disk  radii  were  drawn,  of  which  that  which  passed  through  the 
north  pole  of  the  sun  was  numbered  0°,  and  starting  from  north  the  numbering 
increases  in  the  direction  NESW. 

(6)  THE  38-FOOT  CAMERA. 

Mr.  Fowle  secured  eight  good  negatives  during  and  immediately  after  totality. 
The  times  of  exposure  as  recorded  on  the  chronograph  were  as  follows: 


No. 

Beginning  of  ex- 
posure. 

End  of  exposure. 

Interval. 

1 

2011  45m  IT.  7 

20"  45™  18'.  7 

1     nec. 

2 

20    45    27.7 

1      sec.  (?) 

3 

20    45    38.3 

20    45    40.3 

2.  0  sec. 

4 

20    45    53   9 

4      sec  (?) 

5 

20    46     3.7 

20    46    12.0 

8.  3  sec. 

6 

20    46    23.4 

20    46    31.8 

8.  4  sec. 

f- 
i 

20    46    40.1 

20    46    43.7" 

3.  6  sec. 

8 

20    46    51  .  0 

20    46    52.0 

1.  0  sec. 

a  End  of  totality. 


20  THE    1900    SOLAR    ECLIPSE    EXPEDITION. 

These  negatives  are  excellent,  though  in  detail  they  do  not  equal  those  secured 
with  the  135-foot  camera,  as  was  to  be  expected.  PI.  XXI  is  reproduced  from  the 
exposure  beginning  at  -20h  46m  23s  .4  and  shows  the  coronal  streamers  somewhat 
farther  out  than  Pis.  XII  to  XX  because  of  its  relatively  greater  exposure. 

The  original  negatives  of  the  inner  corona,  especially  those  secured  with  the 
135-foot  focus  camera,  are  so  full  of  detail  that  only  confusion  would  result  from  an 
elaborate  inventory,  but  as  years  go  by  and  future  eclipses  yield  new  points  of  view 
in  regard  to  the  nature  of  the  corona,  it  is  believed  that  these  plates  will  be  of 
increasing  value  for  reference.  In  a  general  view  one  gets  the  impression  that  the 
coronal  streamers  belong  in  two  sets,  respectively  polar  and  equatorial  in  position. 
The  polar  streamers,  like  the  lines  of  force  of  a  bar  magnet,  curve  as  if  to  run  together 
from  the  north  pole  to  the  south.  The  equatorial  streamers  start  in  a  general  way 
radially  and  curve  in  such  a  manner  as  soon  to  stretch  out  farther  and  farther  from  the 
sun  in  a  direction  roughly  parallel  to  the  plane  of  the  ecliptic,  and  exhibit  no  tendency 
ever  to  run  together  from  east  to  west.  Thus  the  polar  streamers  in  numerous 
instances  appear  to  cross  the  equatorial  streamers  before  becoming  too  faint  to  be 
readily  followed. 

On  closer  inspection  it  is  seen  that  the  radial  appearance  of  the  equatorial 
streamers  as  they  emerge  from  the  limb  is  by  no  means  general.  Near  the  large 
prominences  on  the  southwest  limb  some  of  the  streamers  are  much  curved,  while  one 
group  very  remarkably  exhibits  what  seems  to  be  its  origin.  Beginning  at  the 
corner  of  the  large  fan-shaped  spreading  prominence  at  position  angle  265°,  two 
sickle-shaped  coronal  rays,  broad  at  the  base,  start  out  tangent  to  the  limb  and  at 
first  run  toward  the  equal  or,  but  soon  curve  outward  to  join  the  general  direction  of 
the  equatorial  rays,  crossing  over  several  radially  directed  rays  in  their  path,  while 
two  other  noticable  rays  above  the  prominence  seem  to  be  directed  toward  the  same 
origin,  but  from  the  other  side.  Thus  the  four  rays  present  the  appearance  of  a 
doubled-framed  lyre  with  the  center  of  its  bow  just  outside  the  photosphere  and  in  a 
region  of  great  prominences.  Indeed,  it  may  be  said  in  general  that  disturbed  regions 
of  the  corona  seem  to  be  associated  with  large  prominences. 

3.  PHOTOGRAPHY  OF  THE  OUTER  CORONA  AND  OUTLYING  REGIONS 

(a)  The  1\ -foot  focus  camera.  Mr.  Gill  obtained  an  excellent  negative  of 
eighty-two  seconds  exposure,  on  which  the  coronal  streamers  are  seen  to  extend  about 
to  the  image  of  the  planet  Mercury.  Better  results  would  have  been  reached  by 
reducing  the  aperture,  as  the  sky  was  overexposed.  No  stars  fainter  than  the  sixth 
magnitude  are  shown. 

(6)  The  40-inch  cameras.  A  good  coronal  negative  of  about  eighty-two  seconds 
exposure  was  obtained  with  each  of  these  instruments,  in  one  of  which  a  color  screen 


THE   1900   SOLAR    ECLIPSE    EXPEDITION. 


21 


was  employed,  but  without  marked  effect.  The  coronal  streamers  are  seen  to  extend 
about  three  diameters,  but  no  stars  are  shown  fainter  than  the  sixth  magnitude. 

(c)  The  11-foot  cameras.  The  two  negatives  obtained  with  these  instruments 
were  of  unequal  merit.  That  covering  the  region  west  of  the  sun  is  very  good;  the 
other  unfortunately  had  numerous  defects  in  the  film,  and  is  of  little  value.  PI.  I 
(frontispiece)  is  taken  from  a  careful  enlargement  by  Mr.  Smillie  of  the  corona  as 
shown  on  the  first  mentioned  of  these  two  plates,  though  much  of  the  fainter  outlying 
region  is  lost  in  reproduction.  A  glass  positive  from  the  enlarged  negative  shows  the 
whole  region  bounded  by  the  northwest  and  southwest  streamers  to  be  filled  with 
faint  coronal  light  quite  out  to  the  image  of  the  planet  Mercury,  a  sharp  boundary 
line  separating  this  faint  luminosity  from  the  outlying  sky  to  north  and  south. 

In  searching  over  this  plate  for  possible  planetary  objects,  114  stars  were  found. 
The  faintest  is  of  the  8.4  magnitude  as  given  in  Argelander's  Durchmusterung,  though 
not  all  parts  of  the  plate  show  stars  as  faint  as  this,  owing  to  imperfect  focus.  The 
other  negative,  covering  the  region  east  of  the  sun,  showed  only  13  stars,  of  which 
the  faintest  was  of  the  6.3  magnitude. 

In  all,  on  botli  plates,  eight  uncharted  objects  of  starlike  appearance  were  found. 
Four  of  these  are  more  or  less  uncertain  in  their  appearance,  and  deserve  little  notice. 
The  other  four  can  not  be  differentiated  in  appearance  from  the  nearest  stars,  but 
nothing  is  certain  in  regard  to  their  character,  because,  so  far  as  the  writer  is  aware, 
no  other  plates  taken  by  any  of  the  eclipse  expeditions  show  stars  as  faint  as  these,  so 
that  their  reality  can  not  be  verified. 

The  positions  of  these  objects  as  interpolated  on  the  Durchmusterung  charts  of 
1855  is  as  follows: 

EPOCH  1855. 


Right  ascension. 

Declination. 

Magnitude. 

Appearance. 

3h     40"'  22» 

20°     08' 

7.2 

Doubtful. 

3      47     55 

20       21 

5.  0  to  5.  2 

Good. 

3      52     28 

17       50 

6.5 

Good. 

3      58     48 

18      31 

6.1 

Good. 

4      08    08 

19      39 

6.2 

Good. 

4       15     30 

21       49 

7.0 

Doubtful. 

4      31     00 

19      50 

4.5 

Doubtful. 

4      52     00 

20      36 

5.  5  to  6.  0 

Doubtful. 

PI.  XXII,  drawn  from  measurements  of  the  original  negative,  shows  the  position 
of  all  stars  and  starlike  objects  found  in  the  region  west  of  the  sun. 

The  results  obtained  with  the  11 -foot  cameras  make  it  certain  that  no  new 
planets  brighter  than  the  fifth  magnitude  exist,  unless  these  were  hidden  by  the  sun 
or  the  brightest  coronal  regions,  though  it  is  possible  that  there  may  be  such  planets 
fainter  than  the  fifth  magnitude.  If  there  be  any  brighter  than  the  ninth  magnitude, 
they  can  be  photographed  in  future  eclipses  with  cameras  of  this  type. 


22 


TUB    1900    SOLAR    ECLIPSE    EXPEDITION. 


4.  BOLOMETRIC  OBSERVATIONS  OF  THE  CORONA. 

Referring  to  PI.  IX,  and  to  the  accompanying  description  of  the  bolometric 
apparatus,  the  reader  will  recall  that  the  beam  entered  the  spectrobolometer  room 
through  an  aperture  of  variable  size,  that  the  image  was  adjusted  upon  the  slit  jaws 
by  moving  the  large  concave  mirror;  that  the  prism  was  used  sometimes  with  glass 
face  forward,  sometimes  with  silvered  face  forward  as  a  mirror;  that  a  cardboard 
screen  was  at  times  interposed  in  front  of  the  bolometer  to  give  the  zero  reading;  that 
a  glass  screen  was  sometimes  thus  interposed;  and  that  by  a  common  clockwork  the 
prism  could  be  rotated  and  a  photographic  plate  at  the  galvanometer  could  be  moved 
vertically  so  as  to  record  automatically  the  proposed  spectrum  observations. 

Recalling  all  these  arrangements  the  following  data  of  the  observations  may  now 
be  considered: 

BOLOMETRIC  OBSERVATIONS. 

[Current  in  each  bolometer  strip,  0.2  ampere.    Time  of  single  swing  of  galvanometer,  2.0  seconds.    Observer  at  galvanometer,  C.  E. 

Mendenhall.    Observer  at  prism,  C.  G.  Abbot.] 


Aperture  of  cat's-eyc  dia- 
phragm. 

Position  of  slit  with 
reference  to  image. 

Prism  acting  as  mir- 
ror or  as  prism. 

Screen. 

Time  of  obser- 
vation (76th 
meridian  mean 
time). 

Galva- 
nometer 
reading. 

Deflec- 
tion of 
galva- 

iiomt'lcr. 

No.  of 
observa- 
tion. 

20b    42"  (?)• 

40mm 

1 

20     43    00 

25 

2 

Cardboard  insert- 

Immediate 1  y 

25 

3 

ed  in  front  of 

after  above. 

0~.8  X  0°».5  =  0™s.4 

Nearly  tangent  to  di- 

As mirror 

bolometer  after 

20      44     15 

G 

4 

minishing  crescent 

each  deflection. 

Immediately 

6 

5 

of  sun's  limb. 

after  above. 

20      44     35 

G 

G 

Cardboard  screen. 

20      45     15(?) 

60 

00 

7 

At  center  of  moon  — 

do  

No  screen  

20      45     30(?) 

42 

-18 

8 

On  inner  corona 

do  

do  

20      45     45(7) 

47 

-13 

9 

0.2   mm.    beyond 

17°»  x  17«™    =  289™* 

moon's    preceding 

limb. 

At  center  of  moon  

do  

do  

20      45     55(7). 

42 

-18 

10 

On  inner  corona  

.  ...do 

Glass  plate 

20      46     05(?) 

51 

9 

11 

Tangent    to   moon's 

As  prism  for  wave 

No  screen  

20      46     15(?) 

60 

00 

12 

advancing  limb. 

length  0»t.  40. 

36»»x3&»»    =1,225™» 

....do  

do  

do  

20      46     30(?) 

60 

00 

13 

....do  

do  

do  

20      46     40(?) 

60 

00 

14 

....do  

do  

do  

20      46     46(?) 

60 

00 

15 

This  part  of  the  observations  was,   it  will  be  remembered,  in  the  immediate 
charge  of  Mr.  Abbot,  whose  notes  written  five  minutes  after  totality  are  as  follows: 

The  spectrum  was  intended  to  be  turned  from  H  through  to  Jf3,  but  the  clamp  screw  was  not 
turned  by  me  so  that  all  spectrum  readings  were  on  H  in  the  violet.  The  photographic  plate 
may  or  may  not  have  been  exposed  by  C.  E.  M.  He  has  a  vague  idea  that  he  raised  the  shutter. 
It  may  pay  to  develop,  but  all  deflections  are  superposed.  C.  G.  A.  interposed  piece  of  card- 
board in  preliminary  readings,  but  not  during  totality.  The  slit  was  a  very  little  (about  0.2 
millimeter)  west"  of  limb  of  sun,  i.  e.,  beyond  the  advancing  moon.  The  beam  was  a  little  too 
low  on  the  slit  during  the  exposure,  so  that  in  the  moon  deflections  about  one-fifth  of  the  strip 


a  "West"  here  refers  to  position  in  bolometric  house. 


-. 

-A       " 

^ . 

THE    1900    SOLAR    ECLIPSE    EXPEDITION.  23 


was  affected  by  the  radiations  from  the  polar  coronal  region.  The  moon's  path  was  nearly  at 
right  angles  with  the  slit.  C.  G.  A.  could  observe  image  on  white  asbestos  paper.  The  corona 
visible  to  him  extended  out  about  one-half  sun's  diameter  in  all  directions.  He  could  observe  no 
special  structure  or  shape  other  than  circular  in  the  time  he  could  spare  to  see. 

Later  during  the  same  day  an  attempt  was  made  by  a  holographic  process  to 
calibrate  the  apparatus  on  the  solar  spectrum,  but  owing  to  accidental  fogging  of  the 
plate  this  work  was  lost.  The  plate  supposed  to  be  taken  during  the  eclipse  appears 
not  to  have  been  exposed,  as  nothing  showed  on  developing  it. 

The  bolometric  apparatus  was  employed  in  August,  1900,  to  measure  the  radia- 
tions of  the  moon.  Two  days  before  full  moon,  with  the  apparatus  in  every  way  as 
nearly  as  possible  like  that  of  experiment  No.  9  of  the  preceding  table,  but  with 
a  very  humid  and  hazy — indeed,  almost  cloudy — atmosphere,  positive  deflections  of 
55  millimeters  were  obtained  with  the  slit  at  the  center  of  the  moon's  image,  as 
compared  with  a  cardboard  screen  at  the  room  temperature,  and  86  millimeters  as 
compared  with  the  dark  zenith  sky. 

DISCUSSION  OF  THE  BOLOMETRIC  OBSERVATIONS. 

The  observations  1  to  6,  inclusive,  were  made  just  before  totality  on  the  region  of 
sky  where  the  corona  was  shortly  to  be  observed  and  with  an  aperture  of  only  0.4 
square  centimeters.  Reduced  to  the  same  aperture  of  diaphragm  as  that  used  during 
the  coronal  observations  7  to  10,  inclusive,  the  positive  deflections  in  experiments  1 
to  5  would  range  from  28,800  down  to  4,300  divisions.  During  totality  all  the 
deflections  were  negative.  This  was  extremely  disconcerting  to  the  observer  at  the 
galvanometer,  for  in  anticipation  of  considerable  positive  deflections  from  the  corona 
the  spot  of  light  had  been  brought  near  the  zero  of  the  scale,  and  hence  but  little 
room  remained  for  negative  deflections.  Hence  it  was  that  the  cats-eye  diaphragm 
was  not  opened  wider  in  experiments  7  to  10,  inclusive. 

If  we  assume  the  temperature  of  the  dark  moon  to  be  approximately  the  temper- 
ature of  space,"  we  find  on  comparing  the  negative  deflections  observed  in  experiments 
8,  9,  and  10,  that  the  radiation  of  the  corona  caused  a  deflection  of  five  divisions 
[ — 13 — ( — 18)].  The  brightest  coronal  radiation  is  then  very  feeble  indeed  as 
compared  with  the  radiation  scattered  by  our  atmosphere  when  illumined  by  the 
narrow  crescent  of  the  photosphere  even  at  a  single  minute  before  totality. 

The  meaning  of  the  negative  deflections  deserves  explanation.6  It  is  well  known 
that  in  a  room  of  uniform  temperature  bodies  in  general  radiate  the  same  amount  of 
energy  they  absorb,  while  a  warmer  body  radiates  more  than  it  absorbs  in  its  exchanges 
with  a  cooler  body.  In  the  experiment  number  7,  just  before  totality,  the  interposition 

«  See  Memoirs  National  Academy  of  Sciences,  Vol.  Ill,  p.  31,  and  also  Vol.  IV,  Part  2,  Third  Memoir,  pp.  158- 
160. 

6  See  also  Memoirs  of  the  National  Academy  of  Sciences,  Vol.  IV7,  Part  2,  Ninth  Memoir,  pp.  113-116. 


24  THE    1900   SOLAR    ECLIPSE    EXPEDITION. 

of  the  screen  of  cardboard  may  be  regarded  as  completing  a  constant  temperature 
chamber  in  which  the  bolometer  was  inclosed.  If  we  disregard  the  slight  warming  of 
the  bolometer  by  the  electric  current,  it  may  be  said  to  have  absorbed  radiations  from 
the  screen  equal  in  amount  to  the  radiations  which  itself  emitted  toward  the  screen. 
But  when  the  screen  was  removed  in  passing  from  experiment  7  to  experiment  8,  the 
surface  of  the  moon  in  effect  replaced  the  screen  and  became  a  small  portion  of 
the  wall  of  the  chamber  inclosing  the  bolometer.  Owing  to  the  low  temperature 
of  the  dark  moon,  which,  as  the  writer  and  others  have  elsewhere  shown  is  indistin- 
guishable from  the  temperature  of  space,  less  radiation  now  reached  the  bolometer 
from  this  direction.  Hence  the  rate  of  output  of  energy  of  the  bolometer  for  an 
instant  exceeded  the  inflow,  until  with  diminished  temperature  its  radiation  was 
sufficiently  reduced  to  establish  a  new  condition  of  equilibrium.  The  corresponding 
reading  of  the  galvanometer  was  of  course  in  a  negative  direction  from  the  reading 
corresponding  to  the  card  at  room  temperature  which  was  taken  as  zero. 

The  interpretation  of  the  negative  deflections  then  is  that  the  dark  moon  and 
inner  corona  were  effectively  cooler  than  the  room  temperature,  which  was  reasonably 
to  be  expected  in  the  former  case,  but  was  surprising  in  the  latter,  when  the  visible 
intensity  of  the  coronal  radiations  is  considered.  From  a  comparison  of  the  results  of 
several  observers  of  the  eclipses  of  1870,  1878,  and  1898,  it  has  been  concluded  that 
the  average  brightness  of  the  coronal  region  observed  with  the  bolometer  was  about 
equal  to  that  of  the  full  moon.  But  the  bolometric  experiments  of  August,  1900,  as 
given  above,  show  that  a  deflection  of  eighty-six  divisions  was  given  by  the  nearly  full 
moon  as  referred  to  the  dark  sky,  and  a  positive  deflection  of  fifty-five  divisions  as 
compared  with  the  screen  at  room  temperature. 

To  restate  the  matter  in  another  form,  it  has  been  shown  by  the  writer  and  others 
that  the  radiation  of  the  dark  moon  is  practically  indistinguishable  in  amount  from 
that  of  space;  the  reading  on  the  dark  moon  may  therefore  be  regarded  as  the  zero  of 
the  series,"  and  the  same  which  would  have  been  obtained  upon  the  sky  in  the  direc- 
tion of  the  inner  corona  had  the  latter  been  absent.  Referred  to  this  zero  the  radiation 
of  the  inner  corona  produced  a  deflection  of  five  divisions;  that  of  a  card  at  room 
temperature  produced  eighteen  divisions;  that  of  the  full  moon  produced  eighty-six 
divisions,  and  the  diffuse  reflection  of  the  rays  of  the  crescent  sun  in  the  five  minutes 
before  totality  from  4,300  to  29,000  divisions. 

Why  then  should  the  coronal  radiation  of  equal  apparent  brightness  to  that  of 
the  full  moon  be  comparatively  so  feeble  in  heating  effect?  It  might  be  urged  that 
this  is  occasioned  by  the  supposed  rare  gaseous  structure  of  the  corona,  through 

a  The  radiation  of  our  own  atmosphere  depending  upon  its  temperature,  is  superposed  on  that  of  the  moon, 
and  also  on  that  of  each  of  the  other  sources  measured,  hut  being  small  in  amount,  and  as  appears  from  the  equality 
of  the  two  readings  on  the  dark  moon,  presumably  constant  in  the  single  half  minute  of  the  eclipse  observations,  it 
disappears  from  the  differences  of  the  galvanometer  readings,  and  is  hence  neglected.  It  should  be  noted  that  as 
the  sun  is  hidden,  the  diffuse  reflection  of  the  sky  is  not  here  in  question. 


THE    1900    SOLAR    ECLIPSE    EXPEDITION.  25 

which  the  radiations  of  the  bolometer  may  pass  readily  and  reach  the  cold  space 
beyond;  and,  furthermore,  it  might  be  said  that  the  Milky  Way,  known  to  be 
composed  of  numerous  hot  stars,  would  undoubtedly  be  indistinguishable  by  the 
bolometer  from  the  blackest  sky.  To  these  considerations  it  must  be  replied  that  the 
amount  of  radiation  of  the  bolometer  outward  is  a  function  of  its  own  temperature 
only,  and  is  independent  of  the  nature  of  the  objects  it  radiates  upon;  so  that  what  is 
different  in  the  observations  of  the  full  moon  and  the  inner  corona  is  not  the  outward 
radiation  of  the  bolometer,  but  rather  the  amounts  of  radiation  they  send  back  to  it. 
But  if  these  are  to  the  eye  the  same,  why  should  they  be  different  in  heating  effect? 
Obviously  a  natural  explanation  would  be  the  comparative  absence  of  some  invisible 
radiations  in  the  case  of  the  inner  corona,  which  are  present  in  the  case  of  the  moon, 
such  as  would  be  due  to  an  absence  of  infra-red  rays.  In  the  case  of  the  Milky  Way 
cited  above  no  such  conclusion  is  to  be  drawn,  because  the  Milky  Way  is  as  faint  to 
the  eye  in  comparison  with  the  full  moon  as  It  is  feeble  in  heating  effect  upon  the 
bolometer. 

The  conclusion  just  reached  from  experiments  7,  8,  9,  and  10,  namely,  that  the 
inner  corona  appears  to  be  lacking  in  infra-red  rays,  depends  mainly  on  two  assump- 
tions, first,  that  the  experiments  are  substantially  correct;  second,  that  the  average 
visible  brightness  of  the  coronal  region  observed  is  nearly  equal  to  the  average  bright- 
ness of  the  full  moon.  Both  require  confirmation  in  future  eclipses. 

As  regards  experiment  number  11,  the  result  reached  is  irreconcilable  with  the 
others  unless  the  glass,  which  was  small,  failed  of  being  interposed  so  as  wholly  to 
cut  off  the  beam,  or  unless  an  error  of  recording  was  made  so  that  the  reading  should 
have  been  61,  not  51.  The  result  to  be  expected  by  the  interposition  of  the  glass  was 
that  a  small  positive  deflection  would  result,  for  glass  is  opaque  to  the  radiations  of 
the  bolometer,  and  would  have  the  essential  properties  of  a  screen  at  the  room  temper- 
ature, while  at  the  same  time  it  would  transmit  most  of  the  feeble  coronal  radiations, 
which  would  cause  a  slight  positive  deflection. 

Experiments  12  to  15,  inclusive,  are  in  effect  equivalent  to  a  repetition  of  experi- 
ment 7,  for  the  coronal  spectrum  in  the  violet  is  evidently  too  feeble  in  heating  effect 
to  cause  a  deflection,  and  the  prism  would  act  only  as  a  reflector  of  the  radiations  from 
the  walls  of  the  room  to  the  bolometer.  These  experiments  are  then  to  be  regarded 
as  furnishing  four  additional  zero  readings,  and  indicate  by  their  agreement  the  excel- 
lent behavior  of  the  bolometric  appai-atus. 

SUMMARY    AND    CONCLUSION. 

The  expedition  was  generally  successful  in  its  observations. 

The  eclipse  was  attended  with  only  a  moderate  degree  of  darkness.  Telescopic 
observations  indicated  considerable  coronal  detail,  though  less  than  had  been  observed 
in  1878.  Large  prominences  were  present,  and  these  appear  to  have  been  associated 
108A— (M 4 


26  THE    1900   SOLAR    ECLIPSE    EXPEDITION. 

\ 

with  regions  of  coronal  disturbance.  The  equatorial  streamers  were  followed  on 
photographs  to  nearly  4  solar  diameters,  and  were  there  lost  by  reason  of 
diminished  intensity  rather  than  as  appearing  to  end.  The  polar  streamers  were 
conspicuous,  and  curved  in  much  the  same  way  as  the  lines  of  force  of  bar  magnets. 
Excellent  photographs  filled  with  detail  were  secured  with  the  long-focus  cameras, 
and  these,  it  is  hoped,  will  have  greal;  and  increasing  value  for  reference  in  future 
years.  The  135-foot  focus  lens  proved  of  the  highest  value  for  the  purpose  of  eclipse 
photography. 

In  the  bolometric  observations  the  heating  effect  of  the  inner  coronal  radiations 
was  recognized  and  found  unexpectedly  feeble.  The  results  seem  to  indicate  a 
comparative  weakness  of  the  infra-red  portion  of  the  coronal  spectrum,  alike  incon- 
sistent with  the  hypothesis  that  it  radiates  chiefly  by  virtue  of  a  high  temperature,  or 
acts  chiefly  as  a  reflector  of  ordinary  sunlight.  This,  taken  in  connection  with  the 
appearance  of  the  corona,  seem  to  support  the  hypothesis  that  the  principal  source  of 
its  radiations  is  of  the  nature  of  an  electrical  discharge.  The  well-known  polorization 
of  its  outer  portions,  and  the  presence  of  faint  dark  lines  in  the  outer  coronal  spectrum, 
announced  many  years  ago  by  Jaussen  and  confirmed  by  the  photographs  of  Perrine 
in  the  eclipse  of  1901,  prove  that  a  small  portion  of  the  coronal  radiation  is  due  to 
reflected  photospheric  light.  But  the  photographs  of  the  coronal  spectrum  by 
Campbell  in  1898,  and  Perrine  in  1901,  indicate  that  the  principal  part  of  the  coronal 
light  is  not  reflected  sunlight.  Many  are  disposed  to  believe  the  main  source  to  be 
the  incandescence  of  particles  due  to  the  proximity  of  the  hot  photosphere,  but  so  far 
as  the  writer  is  aware  the  spectroscopic  evidence  is  equally  in  accord  with  the 
hypothesis  of  a  glow  electrical  discharge.  An  example  of  such  a  discharge  is  found 
in  the  aurora  of  the  terrestrial  atmosphere,  but  while  we  can  hardly  deny  the 
possibility  of  its  existence  in  the  case  of  the  sun  the  above  observations  do  not  seem 
to  the  writer  to  be  conclusive  on  the  point.  In  any  case  bolometric  observations  at 
future  eclipses  seem  very  desirable  as  furnishing  a  method  of  determining  the  real 
nature  of  the  coronal  radiations. 

The  results  of  the  search  for  intra-mercurial  planets  render  it  improbable  that  any 
of  these  bodies  exist  brighter  than  the  fifth  stellar  magnitude,  but  there  may  still  be 
some  fainter  than  this.  Several  suspicious  objects  were  found  on  one  plate,  but  could 
not  be  confirmed  for  lack  of  a  duplicate  photograph  of  equal  merit.  The  3-inch 
aperture  11 -foot  focus  camera  proved  well  adapted  for  the  search,  and  in  future 
eclipses  intra-mercurial  planets  may  be  photographed  with  similar  apparatus,  if  any 
such  planets  brighter  than  the  ninth  stellar  magnitude  exist. 

In  conclusion,  I  desire  to  express  my  gratitude  for  the  aid  furnished  by  so  many 
in  furthering  the  interests  of  the  expedition,  and  to  each  and  all  of  the  observers  whose 
untiring  efforts  in  preparation  and  rehearsal,  and  whose  skill  and  coolness  during  the 

eclipse,  insured  the  successful  issue. 

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PLATE  VIII. 


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